1. Field of the Invention
The invention relates generally to data processing in oilfield applications. More particularly, the invention relates to processing of data acquired by a downhole tool, such as a formation tester.
2. Background Art
Oil and gas industry uses various tools to probe the formation penetrated by a borehole, to locate hydrocarbon reservoirs and to determine the types and quantities of hydrocarbons. During such a logging operation, a logging tool (such as a formation tester) is lowered into a borehole, either after the well has been drilled or while the well is being drilled. Examples of formation testers in the market include the Modular-Formation Dynamic Tester (MDT®, mark of Schlumberger), and the StethoScope® (mark of Schlumberger).
A formation tester may be lowered into a borehole on a wireline to take measurements after a well has been drilled. See, e.g., Dong et al., “Downhole Measurement of Methane Content and GOR in Formation Fluid Samples,” paper SPE 81481, for a description of a Wire-line-conveyed Formation Tester (WFT) used in obtaining GOR. When a formation tester is part of a drilling assembly, it can perform formation testing while drilling (FTWD). Measurements performed by the FTWD may include pressures of the formations, which are referred to as formation pressure while drilling (FPWD). See, Pop et al., in “Operations Aspects Of Formation Pressure Measurements While Drilling,” paper SPE 92494, for a description of FPWD. In addition to FPWD, FTWD are expanding to include measurements such as optical absorption spectroscopy measurements that can be used to compute the Gas/Oil Ratio (GOR).
FIG. 1 is a general illustration of a drilling rig and a drill string with a downhole logging tool in a borehole. The rotary drilling rig shown comprises a mast 1 rising above ground 2 and is fitted with a lifting gear 3. A drill string 4 formed of drill pipes connected one to another is suspended from the lifting gear 3. The drill string 4 has at its lower end a drill bit 5 for drilling the well 6. Lifting gear 31 consists of crown block 7, the axis of which is fixed to the top of mast 1, vertically traveling block 8, to which is attached hook 9, cable if passing round blocks 7 and 8 and forming from crown block 7, on one hand de-ad line 10a anchored to fixed point 11 and on the other active line 10b that winds round the drum of which 12.
Drill string 4 is suspended from hook 9 by means of swivel 13, which is linked by hose 14 to mud pump 15. Pump 15 permits the injection of drilling mud into well 6, via the hollow pipes of drill string 4. The drilling mud may be drawn from mud pit 16, which may be fed with surplus mud from well 6. The drill string 4 may be elevated by turning lifting gear 3 with winch 12. Drill pipe raising and, lowering operations require drill string 4 to be temporarily unhooked from lifting gear 3; the former is then supported by blocking it with wedges 17 in conical recess 18 in rotating table 19 that is mounted on platform 20, through which the drill string passes. The lower portion of the drill string 4 may include one or more tools, as shown at 30, for investigating downhole drilling conditions or for investigating the properties of the geological formations. Tool 30 may include a formation tester.
Variations in height h of traveling block 8 during drill string raising operations are measured by means of sensor 23 that may be an angle of rotation sensor coupled to the faster pulley of crown block 7. The weight applied to hook 9 of traveling block 8 may also be measured by means of strain gauge 24 inserted into dead line 10a of cable 10 to measure its tension. Sensors 23 and 24 are connected by lines 25 and 26 to processing unit 27 which processes the measurement signals and which incorporates a clock. Recorder 28 is connected to processing unit 27, which is preferably a computer. In addition, the downhole tool 30 may also include a processing unit. The downhole processing unit and/or the surface processing unit 27 may be involved in data acquisition, data processing, and telemetry.
Data obtained by a downhole tool are often in the form of time series. Each time-series data may be referred to as a “channel,” and includes measurements or computations of a particular quantity, typically obtained at regularly-spaced intervals of time. For example, measurement channels from formation testers (such as the MDT®) may include fluid optical densities, fluid fluorescence emission levels, pressures, temperatures, fluid resistivities, and pump motor speeds, collected using sensors within the formation tester such as an optical spectroscopic device, a pressure sensor, and an acoustic sensor. Computation channels from formation testers, for example, may include water fraction, hydrocarbon Gas-Oil Ratio (GOR) and hydrocarbon compositions.
For FTWD (or other while drilling measurements and logging, i.e., LWD or MWD), it is desirable to send selected data uphole in real time via mud pulse telemetry, because drilling operations need to be adjusted in real time based on the measured data. Mud pulse telemetry is a common method used in LWD or MWV operations to transmit log data to the surface. Mud pulse telemetry makes use of the modulations of the pressure of drilling fluid pumped through the drilling assembly. The fluid pressure modulation, however, has an extremely narrow bandwidth and can only transmit data at a rate of a few bits per second (typically, less than 10 bits per second). The bandwidth of mud pulse telemetry is often the bottleneck of a logging operation. Therefore, it is desirable to have methods or systems that can overcome the bandwidth limitation typically encountered in logging operations.